The invention relates to techniques and devices for adjusting the transfer function of an optical device or system by varying the profile of attenuation as a function of wavelength. The invention is particularly useful, for example, in wavelength division multiplexing (WDM), especially in dense WDM (DWDM) communication systems.
According to an inventive aspect, one or more polarization encoding steps impart polarization attributes to a signal as a function of wavelength. The polarization attributes provide characteristics by which it is possible to selectively impart controlled amount of attenuation as a function of wavelength. The encoded signal is applied to polarization sensitive elements that provide differing attenuation as a function of polarization. The result is a practical and effective attenuation profile control, i.e., control of attenuation, as a function of wavelength.
According to a further inventive aspect, a series of tunable birefringent waveplates are stacked with at least one fixed waveplate and polarizing filters, to provide a transfer function that is approximately linear as a function of wavelength. The slope, the range of wavelength of interest, the overall attenuation and the peak position of which are adjustable by tuning the retardation of the tunable birefringent waveplates.